The so-called M-effect differs very much depending upon whether the base metal is silver or copper. Where the base metal is silver, a severing of the silver base ribbon can be achieved in such a way as to comply with most existing time-current curve requirements.
These requirements cannot be met in many instances where the base metal is copper, or an alloy of copper.
The operation of fusible elements under consideration is governed by Fick's laws of diffusion. Therefore a ribbon of a base metal could be severed more rapidly by an overlay metal if the overlay metal could be arranged on both sides of the base metal so to interact with the base metal on two interfaces thereof. However, this method has proven not to be feasible because affixing of the second overlay metal results in melting of the first overlay metal, and initiates the metallurgical reaction of the first overlay metal intended to ultimately sever the base metal. This premature effect of the first overlay metal on the base metal resulting in a premature alloy formation of both metals is also referred-to as ageing of the base metal.